1. Field of the Invention
The invention relates to zinc-containing optical glass materials which have refractive indices nd of from 1.52 to 1.66 and Abbe numbers νd of from 35 to 54.
2. Background Information
During the production of optical glass materials, in addition to an accurate setting of the required optical properties and achieving a high transmission, the production costs and raw materials costs play an important role.
The latter can be achieved in particular by using inexpensive batch raw materials and by glass compositions which require relatively low melting temperatures and therefore entail lower energy costs.
A large proportion of the known glass materials which have the abovementioned optical properties (refractive indices, Abbe numbers) belong to the group of barium flint glasses. To set the desired optical properties and to achieve high transmission, substantially the high-quality and expensive raw materials from the group consisting of the alkaline earth metal oxides (BaO, SrO, CaO) and PbO are used.
Since the glass components BaO and PbO have become the subject of public discussion in terms of the environmental pollution which they cause, the manufacturers of optical equipment need optical glass materials which contain the smallest possible amounts of these components.
Partially or completely substituting one glass component with one or more other glass components does not generally allow the required optical properties to be reproduced. Instead, new developments or far-reaching changes in the glass composition are required.
The patent literature includes a number of documents which already describe glass materials with optical characteristics from the said range and with similar compositions. However, these glass materials exhibit a wide range of drawbacks:
In addition to from 7 to 30% by weight ZnO, the glass materials from Japanese document JP 85-24060 B2, with refractive indices nd of from 1.59 to 1.63 and Abbe numbers νd from 36 to 50, also contain the critical components BaO and PbO, in each case in an amount of from 1 to 21.5% by weight.
The BaO content of from 0.9 to 15.7% by weight which is provided according to SU 1534979 A1 and is absolutely necessary is to be regarded equally critically; these glass materials exhibit a refractive index nd of from 1.577 to 1.634.
Japanese document JP 89-133956 A relates to optical glass materials with refractive indices nd of between 1.51 and 1.59 and Abbe numbers νd of between 39 and 58, which, although only optionally, contain up to 44.06% by weight PbO, up to 25.34% by weight B2O3, up to 19.06% by weight SrO and up to 11.3% by weight CaO. In addition, these glass materials contain relatively large amounts of Al2O3 ranging from 4.45 to 44.10% by weight, which entails an increase in the melting temperature. Also, from 0.08 to 8.0% by weight F is absolutely imperative.
Moreover, the glass materials may contain in total up to 11.1% by weight As2O3 and Sb2O3. When these components are used in such high quantities, increased safety provisions are required during production.
At nd of ≧1.62, the glass materials from document JP 89-308843 A have relatively high refractive indices, while their Abbe numbers are relatively low at νd≦37. In addition to possible high levels of BaO, SrO and CaO, a high PbO content of some 30 to 58% by weight is absolutely imperative.
With an nd of from 1.62 to 1.85, the optical glass materials described in document JP 85-221338 A likewise exhibit relatively high refractive indices, the glass materials containing at least 1 to 50% by weight B2O3 and, if only optionally, high levels of BaO (0 to 50% by weight), SrO (0 to 40% by weight), CaO (0 to 30% by weight) and PbO (0 to 30% by weight). B2O3 reduces the chemical-durability of the glass materials.
This also applies to the optical glass materials described in document SU 97 56 17 which, in addition to a B2O3 content of from 2 to 90% by weight, also include CaO (5 to 10% by weight).
The borosilicate glass materials described in document U.S. Pat. No. 4,562,161 likewise exhibit the said optical range, but contain at least 3% by weight B2O3 and at least 4% by weight TiO2, TiO2 having an adverse effect on the transmission, in particular in the UV range.
German document DE 97 33 50 describes optical glass materials which cover a relatively wide range of refractive indices (nd 1.43 to 1.77) and Abbe numbers (νd 28 to 67) and therefore have broad compositional ranges. In addition to the strikingly high optional content of up to 50% by weight PbO, the glass materials contain from 0.2 to 30% by weight TiO2 and from 1.0 to 12% by weight F.
In addition to the principal component SiO2, the optical glass materials described in document JP 85-122747 contain a relatively high level of from 5 to 30% by weight CaO, and the high Abbe numbers νd which can be achieved lie in the range from 51 to 57.
The cesium-containing optical glass materials described in document DE 22 63 501 C2 likewise cover a range of relatively high Abbe numbers νd of from 50 to 60. The component Cs2O, which is employed in an amount of between 5 and 82.5% by weight, makes the glass material considerably more expensive.
The Al2O3-containing (4–16% by weight) glass materials with a relatively low alkaline metal content (Na2O+K2O 1 to 9.5% by weight) described in document JP 07 06 18 36 A are not used as optical glass materials, but rather for specific magnetic applications which require the components FeO2 and MnO2 which are described.
In the glass material which can be doped with Er2O3 according to EP 0 673 892 A2, which contains from 1.5 to 4% by weight Al2O3, B2O3 is replaced by PbO and/or P2O5 in order to improve the spectroscopic properties as optical intensifiers.
The use of ZrO2, such as for example in the spectacles glass materials described in U.S. Pat. No. 2,433,833 (2 to 15% by weight ZrO2) and GB 2 233 781 A (5.71 to 23.15% by weight ZrO2) and in the TiO2-containing optical glass materials described in EP 0 287 345 A1 (at least 2.3% by weight ZrO2), increases the temperature required to melt the glass materials.
The optical glass materials described in JP 82 22 139 A contain, inter alia, 52 to 75% by weight SiO2, 5 to 25% by weight ZnO and 3 to 15% by weight Li2O. However, the essential feature of these glass materials is not the compositional range, but rather the refractive index which is altered by ion exchange in an NaNO3 molten material.
The document JP 87-12633 A describes optical glass materials with gradually adjustable refractive indices; in these materials, substantially cesium undergoes ion exchange against zinc or against other divalent elements. The glass materials are distinguished in particular by their cesium content of approximately 2.86–66.25% by weight Cs2O.